This invention relates generally to improvements in the operation of natural draft type vaporizers for cryogenic fluids, and more particularly to the elimination or reduction of fogging, and achievement of higher effluent discharge rates, by vaporizer enhanced draft inducement methods.
A vaporizer consists of one or more vertical heat transfer element(s). The areas between the elements are open passages for ambient air to flow downward and in the process become cooled by the vaporizing cryogen on the insides of the elements. As the ambient air cools, it grows more dense. The temperature profile of the downflowing ambient air drops from the ambient air inlet temperature to some much lower temperature in the space between the elements. The exact profile is determined by the heat transfer characteristics, the moisture content and the frost deposited. The velocity of flowing ambient air is determined by the difference in densities of the two columns of air; one in the passage between the elements and the other the column of ambient air outside the vaporizer. This static pressure difference is converted into kinetic energy associated with the acceleration of the ambient air to the velocity in the passage and the friction losses plus the turning loss at the bottom of the vaporizer.
For any given vaporization load, and ambient condition, there is a specific velocity, and therefore mass flow rate that satisfies the balance. When the cold air effluent mixes with the outside ambient air, a fog can be generated where the two air masses join, this fogging problem being exacerbated when the ambient air is characterized by high humidity. This fog can be a nuisance or a hazard. Additionally the cold air discharge itself can be a nuisance or a hazard. The warmer the effluent, the less tendency to produce fog and the lesser the effects on the surrounding.
There is need for a simple, effective way to achieve warmer effluent discharge from the vaporizer, lessening the tendency to produce fog, as well as lessening adverse effects on open areas close to the vaporizer.